Hydraulic fracturing is a process of injecting fluids into an oil or gas bearing formation at sufficiently high rates and pressures such that the formation fails in tension and fractures to accept the fluid. In order to hold the fracture open once the fracturing pressure is released, a propping agent (proppant) is mixed with the fluid and injected into the formation. Hydraulic fracturing increases the flow of oil or gas from a reservoir to the well bore in at least three ways: (1) the overall reservoir area connected to the well bore is increased, (2) the proppant in the fracture has significantly higher permeability than the formation itself, and (3) the highly conductive (propped) channels create a large pressure gradient in the reservoir past the tip of the fracture.
Proppants are preferably spherical particulates that have to withstand high temperatures, pressures, and the corrosive environment present in the formation. If the proppant fails to withstand the closure stresses of the formation, it disintegrates, producing fines or fragments which reduce the permeability of the propped fracture. Early proppants were based on silica sand, glass beads, sand, walnut shells, or aluminum pellets. Silica sand (frac-sand) is still the most prevalent proppant. The use of frac-sand is, however, limited to depths of approximately 1,800 m.
U.S. Pat. No. 4,068,718 relates to a synthetic proppant formed of ‘sintered bauxite’ with an apparent specific gravity greater than 3.4 g/cm3. While the proppant had sufficient compressive strength, the high specific gravity prevented it from being carried far into the fractures. U.S. Pat. No. 4,427,068 describes intermediate strength composite proppants made by mixing calcined diaspore clay, burley clay or flint clay with alumina, ‘bauxite’ or mixtures thereof. The sintered pellets had a specific gravity between 2.7 and 3.4 g/cm3. U.S. Pat. No. 4,522,731 relates to an intermediate strength proppant which is produced by spray agglomeration, featuring a density of less than 3.0 g/cm3. U.S. Pat. No. 4,668,645 to Khaund relates to an intermediate strength proppant made from ‘bauxitic clay’. The proppants described in the Fitzgibbons, Lunghofer and Khaund patents have specific gravities lower than that of the earlier Cooke proppant, and proppants having such lower specific gravities have been used with some success in intermediate depth wells where the stress on the proppant is 5,000 to 10,000 psi.
It is desirable to have still lighter weight proppants which are easier to transport in the fracturing fluid and are therefore carried farther into the fracture before settling out.